1. Field of the Invention
The invention relates to a method of treating water with chlorine dioxide (ClO2).
2. Description of the Related Art
Chlorine dioxide is used in water treatment because of its high bactericidal, virucidal and algicidal activity. Due to the explosive tendency of gaseous chlorine dioxide (c>300 g/m3) and chlorine dioxide solutions (c>26 g/l), chlorine dioxide cannot be stored in compressed form or in solutions of relatively high concentration. Owing to these chemical properties, chlorine dioxide must be produced at the point of use. This is conventionally achieved by mixing basic chemicals in special reactors of chlorine dioxide generation systems. The chemical storage vessels, the metering appliances and also the reactor of the chlorine dioxide systems form a locally linked unit of apparatus which is generally erected in rooms accessed by people.
There are a plurality of methods for synthesizing ClO2 which are used commercially for water treatment, but principally three underlying methods are used. These methods use sodium chlorite (NaClO2) as one of the starting materials. The underlying chemistry of the three methods is explained below. The substances used in these methods are termed starting chemicals, or else reactants.
1. Method Using Sodium Chlorite and Strong Acid
In the first method, a strong acid is used together with sodium chlorite. The strong acid is usually hydrochloric acid or sulphuric acid. When hydrochloric acid is used the reaction stoichiometry is as follows:5NaClO2+4HCl→4ClO2+5NaCl+2H2O
In addition, chlorine dioxide can be formed with the use of sulphuric acid in accordance with the following reaction:10NaClO2+5H2SO4→8ClO2+5Na2SO4+2HCl+4H2O
2. Method Starting from Sodium Chlorite and Chlorine
This method uses gaseous chlorine together with sodium chlorite. The reaction proceeds in two stages, first with the formation of hydrochloric acid.Cl2+H2O→HOCl+HCl
The intermediate, hypochloric acid (HOCl), then reacts with sodium chlorite, forming chlorine dioxide (ClO2).HOCI+HCI+2NaClO2→2ClO2+2NaCl+H2O
The stoichiometric reaction from the two equations isCl2+2NaClO2→2ClO2+2NaCl
3. Method Starting from Sodium Chlorite and Sodium Hypochlorite
In the third method, sodium hypochlorite (NaOCl) is used together with sodium chlorite:NaOCl+HCl→NaCl+HOClHCl+HOCl+2NaClO2→2ClO2+2NaCl+H2O
The synthesis reactions for generating chlorine dioxide are generally carried out in reactors which are operated either continuously or by the batch method.
Two explosion limits must be taken into account in the generation of chlorine dioxide:
more than 6 g of ClO2/l of solution [contact with air] and more than 26 g of ClO2/l of solution [autodecomposition of the aqueous solution]. In the case of the chlorine dioxide syntheses carried out by methods 1 to 3, when use is made of feed chemicals which are passed into the reaction chamber at a concentration of greater than approximately 26 g of ClO2/l of solution, dilution water is added to the reaction chamber in order to bring this concentration below that of spontaneous autodecomposition. The chlorine dioxide solution leaving the reaction chamber which generally contains 20 g of ClO2/l or less is diluted with a further water stream to concentrations of roughly less than 3 g of ClO2/l of solution.
In order that the prior art methods can be operated with satisfactory results with respect to plant safety, chlorine dioxide yield and time-specific production rate, a variety of processing variations are performed, inter alia,                Use of diluted starting chemicals: respective concentrations of the chlorine dioxide solution produced falling below 26 g/l or 6 g/l.        Generating reduced pressure in the reactor by applying a vacuum: reduction of the chlorine dioxide concentration in the gas phase to <300 g/m3.        Generation of reactor overpressure, e.g. by using pressure-retention valves at the reactor outlet: prevention of the formation of a gas phase by exceeding the solubility limit of chlorine dioxide; increasing the yield.        Use of batch methods having long reaction times: increasing the yield when diluted starting chemicals are used.        Use of superstoichiometric acid amounts in the chlorite/acid method and use of superstoichiometric chlorine amounts in the chlorite/chlorine method: increasing the yield.        
Despite the use of these procedures, in the event of incorrect operation of the chlorine dioxide generation systems, e.g. due to loss of dilution water or by failure of pressure control, spontaneous decomposition (explosion) of chlorine dioxide can occur, or chlorine dioxide may, due to leakage or breakage of separation surfaces between the chlorine-dioxide-containing solution and the environment, lead to hazards in areas surrounding the generation systems. The use of diluted starting chemicals which leads to chlorine dioxide solutions of a concentration of less than 6 g/l, and therefore the sacrifice of relatively high time-specific generation rates of the chlorine dioxide systems, also cannot exclude hazard to the surrounding areas of the generation systems by exceeding the MAK value [maximum workplace concentration] of 0.1 ppm in the event of incorrect operation. In order to minimize these hazards, various measures are implemented at the generation systems themselves, and also at the sites where the chlorine dioxide generation systems are erected, e.g. complex servicing work on the generation systems including regular replacement of the reactors, spatially isolated erection sites for the generation systems, forced aeration and air monitoring of the atmosphere of the erection site by continuous gas analysis.
A safe method for environment and people is thus needed, with avoidance of the emission of ClO2 into the environment, in particular into the chambers in which the plant is customarily operated. At the same time, the advantages resulting from the use of concentrated starting chemicals such as, e.g. reduced material transport, higher reaction rate, higher yields, lower reactor volume, should be made utilizable.